This invention relates to membranes for reverse osmosis to be suitably used in desalination of sea water and treatment of waste water, and to a method for making the same.
Various synthetic membranes, in particular high polymer membranes are conventionally used for sea water desalination and waste water treatment through reverse osmosis. They include, for example, membranes made of acetyl cellulose, allyl amine polymer, 4-vinyl pyridine polymer, N-vinyl pyrrolidone polymer, 4-picoline polymer, 4-ethyl pyridine polymer, 4-methyl benzyl amine polymer, styrene polymer, glycydil methacrylate polymer, morpholine polymer, furane polymer. A problem with these conventionally used membranes is that extremely high pressure well over 80 kg/cm.sup.2 must be applied to the saline or waste water for the desalination or waste water treatment. Among them, acetyl cellulose membranes have an additional drawback in that their desalination rate or removal rate is so low (e.g. less than 50%) that several reverse osmotic steps must be repeated to obtain desired results, for example, of desalting sea water. Although allyl amine polymer membranes and 4-vinyl pyridine polymer membranes exhibit high desalination rates (or removal rates) of 90% to 98%, they can transport only a small amount of aqueous solution, for example, 20-400 l/m.sup.2 a day. By contrast, both morpholine and furane polymer membranes are capable of transporting a relatively large amount of aqueous solution, for example, more than 500 l/m.sup.2 a day, but their desalination rates are as low as 30-50%.
In recent years, it has been proposed to use reverse osmotic membranes for separating an ionic substance from a non-ionic substance, for example, separating out sodium chloride from an aqueous solution of urea and sodium chloride. The above mentioned prior membranes cannot be used for this purpose since they primarily pass water and do not pass inorganic salts and non-ionic organic substances.